1. Field of the Invention
The present invention relates to anisotropic conductive adhesive films which are used in electrical and mechanical connections between electronic components such as semiconductor chips and circuit boards, and in particular relates to anisotropic conductive adhesive films which are suitable for use when flip-chip mounting semiconductor chips on polyester-based flexible circuit boards.
2. Description of the Related Art
In the past, anisotropic conductive adhesive films (ACF) have been used to effect electrical and mechanical connections between electronic components such as semiconductor chips and circuit boards, or between circuit boards; recently, however, in such applications as mobile telephones and IC cards in particular, anisotropic conductive adhesive films have come into wide use for flip-chip mounting (COF) IC chips on flexible circuit boards (FPC).
Anisotropic conductive adhesive films generally have a single layer structure, in which electrically conductive particles are dispersed in a single epoxy resin-based insulating adhesive layer. Other known structures include dual-layer structures comprising one insulating adhesive layer in which the electrically conductive particles have been dispersed and the other insulating adhesive layer of the same composition in which they are not so dispersed, or comprising an insulating adhesive layer in which the electrically conductive particles have been dispersed and a layer of that insulating adhesive with one or any components of the composition having been changed.
On the other hand, in mobile telephones, IC cards, and other applications there is a strong demand to keep costs low in order to provide a competitive price. Accordingly, in these applications polyester-based flexible circuit boards tend to be used instead of conventional polyimide-based ones.
However, polyester-based flexible circuit boards have poorer adhesion than polyimide-based ones, because of affection of the surface properties thereof. Conversely, if polyimide-based flexible circuit boards are replaced by polyester-based ones, the adhesion between the electronic component and the circuit board shall lower, owing to the high modulus of elasticity in anisotropic conductive adhesive films in which conventional epoxy resin-based insulating adhesives are employed.
In response to the above drawback, acrylate-based adhesives, which have high adhesion and a low modulus of elasticity, have been tested as insulating adhesives for anisotropic conductive adhesive films. However, another drawback has arisen in that the continuity reliability lowers, despite increased adhesion.
For these reasons, it has not been possible to achieve the required specifications with regard to adhesion and continuity reliability when using conventional anisotropic conductive adhesive films for mounting IC chips and other electronic components on polyester-based flexible circuit boards.
With the foregoing problems of the prior art in view, it is an object of the present invention to realize high adhesion and continuity reliability using anisotropic conductive adhesive films, even when mounting IC chips and other electronic components on polyester-based flexible circuit boards.
The present inventors perfected the present invention by discovering that by using an anisotropic conductive adhesive film when mounting IC chips and other electronic components on polyester-based flexible circuit boards, forming the insulating adhesive layer which constitutes the anisotropic conductive adhesive film into a multiple-layer configuration with one layer on the electronic component side and the other layer on the flexible circuit board side, and by making the modulus of elasticity after the insulating adhesive layer on the flexible circuit board side has been cured lower than the modulus of elasticity after the insulating adhesive layer on the electronic component side has cured, adhesion and continuity reliability will simultaneously increase.
In other words, the present invention provides an anisotropic conductive adhesive film comprising a first insulating adhesive layer and a second insulating adhesive layer whose modulus of elasticity after curing is less than the modulus of elasticity of the cured first insulating adhesive layer, in which electrically conductive particles have been dispersed in at least one of the first insulating adhesive layer and the second insulating adhesive layer.
Moreover, the present invention provides a method for electrically and mechanically connecting semiconductor chips with circuit boards by using the aforementioned anisotropic conductive adhesive film, in which the first insulating adhesive layer is disposed on the semiconductor chip side of the anisotropic conductive adhesive film and the second insulating adhesive layer is disposed on the circuit board side.
Furthermore, the present invention provides a connected structure assembly having a semiconductor chip and a circuit board electrically and mechanically connected thereto by using the aforementioned anisotropic conductive adhesive film, in which the semiconductor chips is bonded to the cured first insulating adhesive layer of the anisotropic conductive adhesive film and the circuit board is bonded to the cured second insulating adhesive layer.